The present invention relates to a sequence structure analysis of a biopolymer using mass spectrometry.
Nowadays, the analysis of the human DNA sequence has been completed, which puts importance on the structure analysis of proteins generated using the genome information, or biomolecules undergoing posttranslational modification for functioning in the cell based on the proteins.
One of the structure analysis means technique widely used is mass spectrometry. Using the mass spectrometers, such as, an ion trap, a Q mass filter, and the time-of-flight (TOF) mass spectrometer, it is possible to obtain information of the sequence of peptides or proteins. The mass spectrometers have high throughput feature, therefore, they have a good connectivity with sample preparation means for separating a sample, such as a liquid chromatography apparatus. Thus, it is valuable for proteomics analysis, especially for high throughput analysis, and hence it finds a wide range of use.
In mass spectrometry, sample molecules are ionized, and injected into a vacuum (or ionized in a vacuum). The motion of the ions in the electromagnetic field is measured, thereby to determine mass-to-charge ratio of the target molecule ions. It is not possible to obtain as far as the internal structure information with only single mass analysis operation, therefore, a method referred to as a tandem mass spectrometry is used. Namely, the sample molecule ions are identified or selected by the first mass analysis operation. These ions are referred to as precursor ions. Subsequently, the precursor ions are dissociated. The dissociated ions are referred to as fragment ions. The fragment ions are further subjected to mass analysis, thereby to obtain information of patterns of the fragment ions. Each dissociation reaction has own dissociation pattern, which enables the judgment of the sequence structure of the precursor ions. In particular, in biomolecule analysis, Collision Induced Dissociation (CID), Infra Red Multi Photon Dissociation (IRMPD), and Electron Capture Dissociation (ECD) are adopted.
In the current protein analysis, the most widely used technique is CID. The precursor ions are kinetically energized, and collided with a gas. The molecular vibrations of the precursor ions are excited by the collision, so that dissociation occurs at weak parts of the molecular chain. Whereas, the method which has recently come into use is IRMPD. The precursor ions are irradiated with an infrared laser beam, and allowed to absorb a large number of photons. This excites molecular vibrations, so that dissociation occurs at the weak parts of the molecular chain. The dissociation by CID or IRMPD occurs the sites named a-x and b-y as shown in FIG. 10, out of the backbone composed of an amino acid sequence. Even the a-x and b-y sites may be difficult to cut according to the kind of the amino acid sequence pattern. Therefore, it is known that complete structure analysis cannot be carried out only with CID or IRMPD. For this reason, a sample preparation pretreatment such as digestion using an enzyme becomes necessary, which inhibits high-speed analysis. Whereas, for the biomolecules which have undergone posttranslational modification, when CID or IRMPD is used, the side chain resulting from the posttranslational modification tends to be lost. The side chain tends to be lost, and hence it is possible to determine the modified molecular species from the lost mass. However, the important information regarding the modification site has been done is lost.
On the other hand, ECD which is another dissociation means does not depend upon the amino acid sequence, whereby one position of the c-z site as shown in FIG. 10 on the backbone of the amino acid sequence is dissociated. For this reason, the protein molecules can be completely analyzed by only the mass analytic technique. Further, ECD has a feature of being less likely to dissociate the side chain, and hence is suitable for the means for study/analysis of the posttranslational modification. For this reason, the technique which has particularly received attention in recent years is this dissociation technique referred to as ECD.
It is known that the electron energy required for effecting the ECD reaction is about 1 electron volt (Frank Kjeldsen and Roman Zubarev: Chem. Phys. Lett., 356 (2002) 201-206). Also as is known, the electron capture reaction is caused even at in the vicinity of 10 eV. With the HECD, a large number of fragment ions are generated in each of which in addition to the c-z site, other sites including the a-x site and the b-y site. For using ECD and HECD differently, the control of the electron energy at a precision of 1 eV or less becomes necessary. It has been shown by the study using FT-ICR that ECD is effective for the protein structure analysis/posttranslational modification analysis.
As described above, CID and IRMPD, and ECD respectively provide different sequence information, and hence they can be used complementarily to each other. As one method, CID and IRMPD are used as the main dissociation means. Then, when a complete analysis is impossible with CID and IRMPD, ECD is used complementarily.
However, at the present time, ECD is implemented only by FT-ICR mass spectrometer, but it is not implemented by an industrially widely used radio frequency mass spectrometer such as a radio frequency ion trap and a Q-mass filter. The reason why ECD has been quickly implemented with FT-ICR is based on the principle of trapping of ions. With FT-ICR, a static electromagnetic field is used for trapping ions. Use of a static electromagnetic field enables the introduction of electrons to the trapped ions with a kinetic energy as low as 1 eV with the ions trapped. Namely, the electrons will not be accelerated by a time depending electromagnetic field.
However, FT-ICR requires a parallel high magnetic field (several T or more) through the use of a superconducting magnet, and hence it is high-priced and large-sized. Further, the measurement time required for obtaining one spectrum is from several seconds to 10 seconds, and about 10 seconds is required for the Fourier analysis necessary for obtaining the spectrum. It cannot be said that FT-ICR requiring a total of about several seconds has a good affinity with a liquid chromatography by which one peak occurs in about 10 seconds. Namely, FT-ICR is disadvantageously difficult to use for the high-throughput protein analysis.
If an expensive FT-ICR is not used, and further, high-throughput ECD can be implemented, a high industrial value can be created. For this reason, there have been made some proposals of a method for implementing ECD without using an FT-ICR. Vachet et al., attempted the implementation of ECD by making an electron beam incident into a three-dimensional radio frequency ion trap (see, e.g., R. W. Vachet, S. D. Clark, G. L. Glish: proceedings of the 43rd ASMS conference on Mass Spectrometry and Allied Topics (1995) 1111). However, the incident electrons are heated at a high speed by a radio frequency electric field, and lost in the outside of the ion trap. For this reason, the implementation of ECD has not been reached.
In recent years, the following three methods for implementing ECD without using an FT-ICR have been proposed.
A first method (method A) is the method schematically shown in FIG. 11. A Penning trap static electromagnetic field ion trap composed of a quadrupole static electric field 31 and a static magnetic field 11 is used. A large number of electron beams 29 are trapped in the inside of the Penning trap. The electrons are trapped in the r direction in such a manner as to wind around the line of magnetic force of the static magnetic field 11. Further, the electrons are trapped in the z direction by the z direction component of the static electric field 31. In order to trap electrons having negative charge, the electric potentials on the opposite sides along the z direction are set at a negative potential with respect to the center of the trap. Precursor ions 1 generated at an ion source 16 are made incident as indicated by an arrow 36 upon the electron beams 29 trapped in this manner, and are collided with the electron cloud, thereby to cause the ECD reaction (see, e.g., T. Baba, D. Black and G. L. Glish: 51st ASMS Conference on Mass Spectrometry and Allied Topics, Montreal, Canada (2003) MPK227/ThPJ1 165). The fragment ions generated in the reaction are ejected as indicated by an arrow 37 to be identified by means of a mass analysis means 17.
A second method (method B) is schematically shown in FIG. 12. Precursor ions 1 are trapped in a Penning trap composed of a static magnetic field 32 and a static magnetic field 11. In order to trap positively charged precursor ions, the electric potentials of the opposite sides along the z direction are set at a positive potential with respect to the center of the trap. The precursor ions 1 trapped therein are irradiated with an electron beam 29 (see, e.g., T. Baba, D. Black and G. L. Glish: 51st ASMS Conference on Mass Spectrometry and Allied Topics, Montreal, Canada (2003) MPK227/ThPJ1 165). The electrons reach the precursor ions 1 along the line of magnetic force in such a manner as to wind around the line of magnetic force of the magnetic field (11). The fragment ions generated by the ECD reaction are ejected as indicated by an arrow 37, and identified by means of the mass analysis means 17. In FIGS. 11 and 12, the lines 31 and 32 representing the static electric fields are actual static electric fields, and hence they are shown in solid lines.
A third method (method C) is a method using a three-dimensional radio frequency ion trap as shown in FIG. 13. The electron beam 29 is made incident through a hole made in a ring electrode of the three-dimensional radio frequency ion trap. At this step, a magnetic field 11 is applied in the electron incident direction, so that the electrons are injected to the precursor ions 1 present at the center of the ion trap with high efficiency (see, e.g., I. Ivonin and R. Zubarev: 51st ASMS Conference on Mass Spectrometry and Allied Topics, Montreal, Canada (2003) ThPE057). The fragment ions are analyzed by use of the same three-dimensional radio frequency ion trap, and identified by the ion trap mass spectrometry which is a conventional method.
In FIG. 13, the pseudopotential describing the three-dimensional radio frequency ion trap potential is shown in dotted lines 33. The pseudopotential is the quasi potential formed as the temporal average by the radio frequency electric field, and can be considered with the image described in terms of the static electric field as the approximation. However, in actuality, the effects of the variable electric field occur as micromotion, radio frequency heating, and the like in the movement of the charged particles due to the radio frequency.
The foregoing three methods A, B, and C have been disclosed as the proposals of the principles. At the present time, the ECD reaction has not yet been proved.